Separators are an integral part of the performance, safety, and cost of lithium-ion batteries. During normal operation, the principal functions of the separator are to prevent electronic conduction (i.e., short circuit or direct contact) between the anode and cathode while permitting ionic conduction by means of the electrolyte. For small commercial cells under abuse conditions, such as external short circuit or overcharge, the separator is required to shutdown at temperatures well below those at which thermal runaway can occur. This requirement is described in Doughty. D, Proceedings of the Advanced Automotive Battery Conference, Honolulu, Hi. (June 2005). Shutdown results from the collapse of pores in the separator caused by melting and viscous flow of the polymer, thus slowing down or stopping ion flow between the electrodes. Nearly all lithium-ion battery separators contain polyethylene as part of a single- or multi-layer construction so that shutdown often begins at about 130° C., the melting point of polyethylene.
Separators for the lithium-ion market are presently manufactured through the use of “dry” or “wet” processes. Celgard LLC and others have described a dry process, in which polypropylene (PP) or polyethylene (PE) is extruded into a thin sheet and subjected to rapid drawdown. The sheet is then annealed at 10-25° C. below the polymer melting point such that crystallite size and orientation are controlled. Next, the sheet is rapidly stretched in the machine direction (MD) to achieve slit-like pores or voids. Trilayer PP/PE/PP separators produced by the dry process are commonly used in lithium-ion rechargeable batteries.
Wet process separators composed of polyethylene are produced by extrusion of a plasticizer/polymer mixture at elevated temperature, followed by phase separation, biaxial stretching, and extraction of the pore former (i.e., plasticizer). The resultant separators have elliptical or spherical pores with good mechanical properties in both the machine and transverse directions. PE-based separators manufactured this way by Toray Tonen Specialty Separator, Asahi Kasei Corp., SK Innovation Co., Ltd., and Entek® Membranes LLC have found wide use in lithium-ion batteries.
More recently, battery failures arising in commercial operation have demonstrated that shutdown is not a guarantee of safety. The principal reason is that, after shutting down, residual stress and reduced mechanical properties above the polymer melting point can lead to shrinkage, tearing, or pinhole formation. The exposed electrodes can then touch one another and create an internal short circuit that leads to more heating, thermal runaway, and explosion.
In the case of large format lithium-ion cells designed for hybrid or plug-in hybrid applications (HEV, PHEV), the benefits of separator shutdown have been openly questioned because it is difficult to guarantee a sufficient rate and uniformity of shutdown throughout the complete cell. This issue is described in Roth, E. P., Proceedings of Lithium Mobile Power Conference, San Diego, Calif. (October 2007). Many companies are focused, therefore, on modifying the construction of a lithium-ion battery to include (1) a heat-resistant separator or (2) a heat-resistant layer coated on either the electrodes or a conventional polyolefin separator. Heat-resistant separators composed of high temperature polymers (e.g., polyimides, polyester, polyphenylene sulfide) have been produced on a limited basis from solution casting, electrospinning, or other process technologies. In these cases, the high polymer melting point prevents shutdown at temperatures below 200° C.
U.S. Patent Application Pub. No. US 2012/0145468 describes a freestanding, microporous, ultrahigh molecular weight polyethylene (UHMWPE)-based separator that contains sufficient inorganic filler particles to provide low shrinkage while maintaining high porosity at temperatures above the melting point of the polymer matrix (>135° C.). Such freestanding, heat resistant separators have excellent wettability and ultralow impedance, but they do not exhibit shutdown properties because of the high loading level of the inorganic filler.
U.S. Pat. No. 7,638,230 B2 describes a porous heat resistant layer coated onto the negative electrode of a lithium-ion secondary battery. The heat resistant layer is composed of an inorganic filler and a polymer binder. Inorganic fillers include magnesia, titania, zirconia, or silica. Polymer binders include polyvinylidene fluoride and a modified rubber mixture containing acrylonitrile units. Higher binder contents negatively impact the high rate discharge characteristics of the battery.
U.S. Patent Application Pub. Nos. US 2008/0292968 A1 and US 2009/0111025 A1 each describe an organic/inorganic separator in which a porous substrate is coated with a mixture of inorganic particles and a polymer binder to form an active layer on at least one surface of the porous substrate. The porous substrate can be a non-woven fabric, a membrane, or a polyolefin-based separator. Inorganic particles are selected from a group including those that exhibit one or more of dielectric constant greater than 5, piezoelectricity, and lithium ion conductivity. Selected polymer binders are described. The composite separator is said to exhibit excellent thermal safety, dimensional stability, electrochemical safety, and lithium ion conductivity, compared to uncoated polyolefin-based separators used in lithium-ion batteries. In the case of certain polymer binders mixed with the inorganic particles, a high degree of swelling with an electrolyte can result in the surface layer, but rapid wetting or swelling is not achieved in the polyolefin substrate.
In the latter two of the above approaches, there is an inorganic-filled layer that is applied in a secondary coating operation onto the surface of an electrode or porous substrate to provide heat resistance and prevent internal short circuits in a battery.